ARQ parameter negotiation in a data packet transmission system using link adaptation

ABSTRACT

A method for transmitting data packets between a transmitter and a receiver unit in a communication system is described. For transmitting data packets a transmission mode is selected from a plurality of available transmission modes and an automatic repeat request for retransmission is used. Therefore for each available transmission mode a channel parameter based on link quality is calculated and a transmission capacity parameter is determinated. The state of the automatic repeat request control window for at least one transceiver is identified. For each available transmission mode an estimation of the user quality value based on the channel parameter, the transmission capacity parameter and the state of the automatic repeat request control window from at least one transceiver is estimated. The transmission mode that provides the best user quality value is selected.

The present invention relates to a communication system, which transmitsdata packets. In particular data packets are transmitted with atransmission mode selected from a plurality of available transmissionmodes in that communication system.

In communication systems, data packets are transmitted over a physicallink between different transceivers. Such a structure, as for examplestandardized by the International Standard Organization (ISO), is thereference model of open data interconnections (OSI) [Bertsekas, DimitriP.: “Data Networks”, 2^(nd) ed., Prentice Hall, 1992]. Each transceiver,for transmitting as well as for receiving data packets, is characterizedas having several layers, whereas the both lowest layers are theData-Link-Control-Layer (DLC-Layer or Layer 2 or Convergence Layer) andthe Physical-Interface-Layer (PHY-Layer or Layer 1). ThePhysical-Interface-Layer is the lowest one and provides data packettransmission between the different transceivers over the physical link.

In existing communication systems, different techniques may be employedto transmit data packets between transceivers over the physical link. Itis a widely used method to allocate several transmission time periods ofa transmission frame to several transceivers. In contrary to wiredcommunication systems, in the wireless communication systems, as forexample an EDGE systems, the reliability of data transmission stronglydepends on the radio link quality on the physical link. For exampleburst disturbance in radio link caused by co-channel interference andmulti-path fading introduces a drastic variation of the link quality.

As it is known from WO9913304 a selection method for all availabletransmission modes is described, where a transmission mode is defined asa combination of a coding rate and a modulation scheme. Each combinationof a modulation and coding schemes is based on using measured linkquality parameters to determine which combination provides the best userquality. Based on Eq.1 it is possible to estimate how a change ofmodulation or channel coding scheme would effect the user quality, asfor example the data throughput S_(i). Base on this estimation atransmission mode can be selected that provides the best user quality.S _(i) =R _(i)*(1−BLER _(i))  Eq.1

For each transmission mode i, the maximum data rate R_(i) and the datablock error rate BLER_(i) are given. Based on this assumption themaximal throughput T_(i) can be calculated with equation Eq.1 for eachtransmission mode i. The throughput for all available transmission modesin the system then will be compared. The mode with the maximalthroughput is selected as the suitable transmission mode fortransmitting the data blocks.

As it is known in wireless communication systems, for example shown inTable 1 [Jamshid Khun-Jush: “Structure and Performance of the HIPERLAN/2Physical Layer” Procedures VTC'99 FALL, 1999] a coding rate and amodulation scheme is allocated for the wireless data transmission overthe physical link in the PHY-Layer of a transmitting transceiver. Todecrease the influence of link quality variations on the datatransmission, or more detailed onto the link throughput, in todayexisting wireless communication systems (e.g. HIPERLAN type 2, IS-136and EDGE), the Physical Layer uses various transmission modes. Such aselection of various transmission modes is often called an adaptationscheme. For example, based on link quality measurements, e.g. thecarrier to interference (C/I) ratio, a transmission mode is selectedfrom a list of transmission modes available in that communicationsystem. As a result the link throughput can be maximized, when acombination is selected as a function of the radio link quality.

TABLE 1 Transmission Modulation Coding Physical layer mode scheme ratebit rate 1 BPSK ½  6 Mbps 2 BPSK ¾  9 Mbps 3 QPSK ½ 12 Mbps 4 QPSK ¾ 18Mbps 5 16QAM   9/16 27 Mbps 6 16QAM ¾ 36 Mbps 7 64QAM ¾ 54 Mbps

For error sensitive services in data transmission systems alltransmitted data packets, further also often named as protocol dataunits (PDU's), have to be correctly received by the receiver. Thereforeerroneous transmitted data packets have to be detected and retransmittedby the transmitter again. To detect the erroneous transmitted datapackets, binary Cyclic-Redundancy-Check (CRC) codes are increasingly inuse. Based on the CRC code result, the receiver notifies the transmitterwith an Automatic-Repeat-Request (ARQ) feedback acknowledgment whetherthe transmitted PDU's have been successfully received or not. Theerroneous ones are then retransmitted. In general, three basicretransmission mechanisms, Stop-and-Wait ARQ, Go-back-N (GbN-) ARQ andSelective Repeat (SR-) ARQ, are considered in most data transmissionsystems. In the case of using SR-ARQ, the PDU's are transmittedcontinuously. The transmitter re-transmits only those PDU's, which aredetected as to be erroneous. Since ordinarily PDU's must be delivered tothe user in a correct order, a buffer is provided at the receivingtransceiver, to store the error free received PDU's and the number ofdetected erroneous PDU's. When the first negatively acknowledged PDU issuccessfully received, the receiver than releases the error-freereceived PDU's in a consecutive order until the next erroneouslyreceived PDU is encountered. In the transmitter the buffer must beprovided to store these PDU's which are transmitted until receivingpositive acknowledgements. The buffers in the transmitter and receiverare further referred as ARQ-control-window for the transmitter andreceiver, respectively.

But in today existing communication systems for transmitting datapackets, the ARQ mechanism operates on the DLC layer in a transceiver.This ARQ mechanism is constrained with a limited ARQ control window, dueto a limit of processing power, a limit of memory size and a lowerprotocol overhead. Therefore the transmitter can only send so many PDU'sthat the ARQ window allows. When the link quality of the physical linkis very low, which also results in erroneous transmitted data packets, alot of PDU's has to be retransmitted. In consequence the buffer of theARQ control window in the transmitting transceiver could become blockedand the throughput is reduced. In this case the maximal data rateprovided by a transmission mode can not be utilized. Therefore equationEq.1 is not suitable to optimize the data throughput of radio links,Eq.1 shows only what could be achievable in ideal systems.

It is therefore an object of the invention to provide a method thatovercomes the problem and thereupon increasing the user quality value ofa real communication system.

This is achieved by teaching of claim 1.

In one embodiment, it is advantageous to determine the transmissioncapacity parameter at least by the maximum data rate R_(maxi) providedin each available transmission mode.

In one embodiment, the state of the automatic repeat request controlwindow is determined by the parameters of the automatic repeat requestcontrol window from at least the transmitting transceiver or thereceiving transceiver to estimate the throughput of a real system, whichespecially leads to an optimized overall throughput.

In one embodiment, it is useful to describe the quality value by theuser data throughput. The user data throughput then bases on theprotocol data unit error rate, the maximal data rate, the transmissioncapacity and the state of the automatic repeat request control windowfrom at least one transceiver.

Further, it is advantageous to use the novel method for a radio packetdata system, where the reliability of data transmission strongly dependson the radio link quality on the physical link, e.g. through theinfluence of co-channel interference and multi-path fading in the radiolink.

In the following the invention will be further described according tothe figures and by means of examples. The following figures show:

FIG. 1 a: block diagram of a communication system for data transmissionwith two transceivers;

FIG. 1 b: reference model of a communication system for datatransmission with two transceivers;

FIG. 2: transmission capacity reserved for the transmitter within atransmission frame.

FIG. 3 a-c: diagrams of the performance of user quality values underdifferent preconditions;

FIG. 4: flow chart of a transmission mode selection method for datapacket transmission;

FIG. 5 a: automatic-repeat-request window for a transmitter unit;

FIG. 5 b: automatic-repeat-request window for a receiver unit.

FIG. 1 a shows schematic a block diagram with two transceivers 1, 2within a communication system. Both transceivers include a memory part 1a and 2 a for storing parameters, a controlling part 1 d and 2 d, and areceiver part 1 b, 2 b and a transmitter part 1 c, 2 c for a radiocommunication via an air interface 3. As an alternative, FIG. 1 b showsa part from the above mention OSI reference model of the samecommunication system as shown in FIG. 1 a with these two transceivers 1and 2, usable for transmitting and receiving data packets via the airinterface which is named as the physical link 3 in the context of thisreference model. Based on FIG. 1 b, the invention will be furtherdescribed, where a user1 uses the transceiver 1 as a transmitter and auser2 uses the transceiver 2 as a receiver. The transmitter 1 includes aDLC-Layer 12 for transforming data from a higher Layer m into protocoldata units PDU for the transmission. The DLC-Layer 12 includes anARQ-control-window for a feedback acknowledgment to control the correcttransmission of the PDU's. The PHY-Layer 13 provides different codingand modulation schemes for the transmission of the data packets over thewireless physical link 3. The data packets are transmitted over thephysical link 3 in transmission frames L, as shown in FIG. 2. Eachtransmission frame L includes several consecutive data packetsPDU₁-PDU_(N) within a time slot b.

The physical layer 13 provides different coding and modulation schemesto overcome the above described problem causes from the variations oflink quality. A method for selecting one transmission mode out of agroup of available transmission modes is provided at least in one of thetransceivers 1 and 2. Together with the link quality parameter from thephysical link 3 the user quality for each transmission mode can beestimated.

FIG. 5 a and FIG. 5 b show the automatic-repeat-request-control-windowsfor the transmitter 1 and the receiver 2, which have in contrary toassumptions in the prior art a limited size. The negotiated maximumARQ-control-window sizes in the transmitter 1 and receiver 2 are definedas TxWmax and RxWmax respectively. For both ARQ-control-windows, anupper border TxToW and RxToW and a lower border TxBoW and RxBoW aredetermined. The upper borders are determined through the sequencenumbers of the latest transmitted and correctly received data packetsPDU t+n and PDU r+m. The lower borders are determined through thesequence numbers of the oldest not acknowledged and not correctlyreceived data packets PDU t and PDU r.

As will be mentioned again the existing state of the art solutionsestimates the throughput only on the base of the maximum data rate andthe data block error rate. Therefore it could be assumed thatlimitations of a ARQ control windows, which normally occurs in realsystems are not regarded. The overall throughput in a real system islower as in the idealized system, due to transmission overheads andlimited ARQ-control-windows. In FIG. 3 a the performance of a realsystem is shown in comparison to that one of assuming ideal conditions.The solid line shows for the transmission modes Mode 3 to Mode 7 theideal performance of the overall throughput under the conditions ofunlimited ARQ windows, whereas the dashed lines show, for the sametransmission modes Mode 3 to Mode 7, the real performance of thethroughput by regarding the limited ARQ-window. Wherein the dashed linesin FIG. 3 a shows the complete throughput for all transmission modes,the solid line is the sum of parts of the throughput for differenttransmission modes, named as the overall throughput. As a function ofthe carrier to interference ratio C/I one of the transmission modesMode3 to Mode 7 is selected, depending from which mode a higherthroughput can be achieved. Point a to d represents the equivalentC/I-values, where a transmission mode has to change under idealconditions, whereas point a′ to d′ are the real points for changingbetween different modes. In that regard a performance loss in theoverall throughput causes in the real system, as shown in FIG. 3 boccurs, if the selection of the physical transmission mode is performedin terms of the idealized throughput curve. For example, the idealizedcurve shows that the transmission mode has to be changed fromtransmission mode Mode 6 to Mode 7 at point d, when the C/I-ratio islarger than 20 dB. But the real curve shows that the mode 7 isrecommended at point d′, if C/I is larger than 24 dB. Thus the systemprepares a reversal at 20 dB which results in a reduction of throughputfrom point x′ to x″ at the 20 dB point. In total a throughput loss inthe real system is caused for C/I values between 20 dB and 24 dB, as canbe seen in FIG. 3 b. There the best achievable throughput is followingthe dashed line from point x′ to d′, whereas the state of the artsolution following the solid line from point x′ to point d′ via thepoint x″. In FIG. 3 b it is obviously that a reduction of the overallthroughput also occur after the points a-c.

The preferred method for a selection of a transmission mode, out of allavailable transmission modes, will be further described in more detailby explanation of the flow chart in FIG. 4. The selection of atransmission mode can be done either in the transmitter 1 or thereceiver 2. When the selection is performed in the receiver, theselected mode should be transmitted to the transmitter, which then usesthe selected mode for transmission the data packets. After starting theprocess with step 110, in a first step 112 several preconditions have tobe set. The total number N of all available transmission modes in thatcommunication system is determined and to each of them a transmissionparameter Rmaxi and an estimated link quality parameter C/I areallocated. Also the transmission time b reserved for the transmitter andthe duration L of the transmission frames is determined. Further thestate of the ARQ-control-window from at least one transceiver isidentified. Thereafter, in step 114, the flow parameter i for thefollowing loop is set to i=1 and the value for the throughput to T=0. Inthe decision box 116 that value i has to be compared with the abovedetermined N. If i<N the following loop 116-130 is running. Therefore inthe first step 118 of the loop, the C/I is requested from the memory 112and then 120 mapped to PDU error rate for the transmission mode i. Then,in step 122, the transmission parameter R_(maxi), the reservedtransmission time b, the duration of the transmission frame L and thestate of the ARQ-control-window R_(w) is read from the correspondingmemories 112. As a result of the next step 124 the user quality value isestimated, e.g. the throughput is estimated under the premise ofequation Eq.2, which will be later described in more detail. In the nexttwo steps 126 and 128 there is an update of the throughput T to T_(i),and the transmission parameter R is updated to Rmaxi, if the throughputT_(i) for the actual transmission mode i is higher than any former T.Then i is countered by i+1 and the loop works again for the nextavailable transmission mode, until i is larger than N. If the conditionT>N is fulfilled, in step 132 the parameter list for T and R is readfrom the memory and delivered to the physical layer of the transmittingtransceiver 1. The physical layer then choose the transmission mode,which has the maximum data rate R and uses it for the data transmissionsin the next transmission frame 134. Finally the process can be restartedfor sending further data packets 138 and for example after apredetermined delay time or after detecting that the parameters used inequation 2 have been significantly changed. Else where the process isfinished 138.

The main step 124 of the preferred method for selecting a transmissionmode is now described in more detail. In this selection method the datathroughput of each transmission mode i is calculated based on equationEq.2:T _(i)=Min{R _(w) , R _(max,i) *b/L}*(1−PER _(i))  Eq.2

Where T_(i) is the data throughput for the transmission mode i andPER_(i) is the PDU error rate for the transmission mode i at theconsidered radio link quality. R_(max,i) means the maximal data rate ofthe physical transmission mode i, and R_(w) represents the state of theARQ window either in the receiver or in the transmitter unit, promisedon the DLC layer. b the transmission time reserved for a transceiver fortransmitting data packets within a transmission frame length L. Thevalue of R_(max,i)*b/L represents the transmission capacity for atransmission mode i.

It is the advantageous feature of the invention to follow the state ofthe ARQ-control-window either in the receiver or the transmitter byestimation the term Min{R_(w), R_(max,i)*b/L} in Eq.2, where the maximaldata rate promised R_(w) on the DLC layer must be estimated based onARQ-control-window fullness and ARQ acknowledgements.

The estimation of the state of theautomatic-repeat-request-control-window leads to the achievable datarate R_(w) as will be now described for the two alternative preferredembodiments.

In the first embodiment the state of the ARQ-control-window in thetransmitter 1 will be gathered to determine the maximum data rate of theDLC-Layer 12. On the DLC-Layer of the transmitter 1 data packets fromhigher layers m must be reconstructed to Protocol Data Units (PDU) withsequence numbers t before transmission. The ARQ-control-window in thetransmitter is normally used to control PDU retransmissions. TheARQ-control-window size TxWmax is the maximal number of PDU's that havebeen transmitted and are waiting for acknowledgements from the receiver2. The bottom of the ARQ-control-window TxBoW is the oldest sequencenumber not yet acknowledged by the receiver 2. The top of theARQ-control-window TxToW is the newest sequence number not yetacknowledged by the receiver 2. The number of PDU's to be retransmittedNt in the ARQ-control-window can be determined after receivingacknowledgements. Therefore the maximum data rate on the DLC layer inthe transmitter can be estimated with:R _(w)=(N _(r) +TxWmax+TxBoW−TxToW)/L  Eq.3

The second embodiment takes into account the state of theARQ-control-window from the DLC-Layer 22 in the receiver 2. Here theARQ-control-window is normally used to buffer a number of PDU's that arenot received in order and to deliver the PDU's in sequence to the higherlayers. The ARQ-control-window size RxWmax is the maximal interval ofsequence numbers that are eligible for reception. The bottom of theARQ-control-window RxBoW is the oldest sequence number expected by thereceiver. The top of the ARQ-control-window RxToW is the newest sequencenumber received by the receiver. The number Nr of PDU's to beretransmitted in the ARQ-control-window can be countered based on PDU'slacked between RxBoW and RxToW. So the maximum data rate promised on theDLC layer in the receiver can be estimated with:R _(w)=(N _(r) +RxWmax+RxBoW−RxToW)/L  Eq.4

Finally in FIG. 3 c the simulated results of the overall throughput byusing one of the preferred embodiments are shown. If the C/I valuereaches point x′ the transmission mode Mode 6 will not change to Mode 7,the system first changes to mode 7 close to point d′ when using thepreferred embodiment on the base of equation Eq.2.

As already outlined, the comparing of FIG. 3 b with FIG. 3 c animprovement of the overall throughput can be recognized by using theequation Eq.2 under the premiss of the state of the ARQ-control-windowfrom the receiver 1 or the transmitter 2. It is distinct that theselection criterion based on equation (2) is more reliable than thatusing equation (1) and guarantees the best throughput of the system indifferent radio link qualities (C/I).

Thus, the present invention increases the overall throughput of atransmission system and leads to an optimized system with bestperformance. In the following a preferred embodiment of a transceiverfor transmitting and/or receiving data packets over a physical link in acommunication system is briefly described, where the above describedmethod is implemented. A controlling part 1 d, 2 d, as shown in FIG. 1a, is needed at least in one transceiver, to perform the selectionmethod, as for example described in FIG. 4. That transceiver integratesa calculator for calculating a channel parameter based on the linkquality and a determinator for determinating a transmission capacityparameter for each available transmission mode i. An identifer foridentifing the state of a automatic repeat request control window inthat transceiver is included. Although the controlling part includes anestimator for estimating user quality value for each availabletransmission mode based on the channel parameter, the transmissioncapacity parameter and the state of the automatic repeat request controlwindow from at least one transceiver. Finally the controlling partincludes a selector for selecting a transmission mode that provides thebest user quality value. The above described controlling part 1 d, 2 dis used as a synonym for all kind of hardware, that can be used inmobile terminals for data processing and controlling purposes. Thereforegeneral purpose processing devices like so called micro processors,dedicated programmable hardware like so called digital signal processorsas well as hardware programmable logic circuits like ApplicationSpecific Integrated Circuits (ASICs) should be covered by the termprocessing device. Due to certain constraints like computing power,integration size, availability etc. up to now it was common todistribute functions like processing and controlling to more than onedevice. Therefore a person skilled in the state of the art should beaware that processing device also means a set or any combination ofmicroprocessors, digital signal processors, ASIC's etc.

Furthermore it has be mentioned again that the invention is notrestricted to the specific embodiments and examples described in thepresent invention. That means, that the above described method canimplemented in any data packet transmission system, where the abovedescribed problems can be solved by regarding the influence of the realARQ-control-window size from at least the transmitting or receivingtransceiver. That is, on the basis of the teaching contained in thedescription, various modifications and variations of the invention maybe carried out.

1. A method for transmitting data packets between two transceivers in acommunication system, wherein a transmission mode for transmitting thedata packets is selected from a plurality of available transmissionmodes and an automatic repeat request for retransmission is used,comprising the steps of: calculating, for each available transmissionmode, a channel parameter based on the link quality; determining, foreach transmission mode, a transmission capacity parameter; identifing astate of an automatic repeat request control window for at least onetransceiver; estimating a user quality value for each availabletransmission mode based on the channel parameter, the transmissioncapacity parameter, and the state of the automatic repeat requestcontrol window from at least one transceiver; and selecting atransmission mode of the plurality of available transmission modes, theselected transmission mode providing the best user quality value;wherein the transmission capacity parameter is determined by a maximaldata rate (R_(maxi) ) provided in each available transmission mode, atransmission (b) and transmission frame (L) length provided fortransmitting data packets.
 2. The method of claim 1, wherein the channelparameter for each available transmission mode is a protocol data uniterror rate (PER_(i)).
 3. The method of claim 1, wherein, in atransceiver that transmits data packets, the state of the automaticrepeat request control window R_(w) is determined by an oldest protocoldata unit sequence number TxBoW, a newest protocol data unit sequencenumber TxToW and a number of data packets Nt in a window to beretransmitted.
 4. The method of claim 1, wherein, in a transceiver thatreceives data packets, the state of the automatic repeat request controlwindow R_(w) is determined by an oldest protocol data unit sequencenumber RxBoW expected by a receiver unit, a newest protocol data unitsequence number RxToW received by the receiver unit and a number of datapackets Nr in a window to be retransmitted.
 5. A method for transmittingdata packets between two transceivers in a communication system, whereina transmission mode for transmitting the data packets is selected from aplurality of available transmission modes and an automatic repeatrequest for retransmission is used, comprising the steps of:calculating, for each available transmission mode, a channel parameterbased on the link quality; determining, for each transmission mode, atransmission capacity parameter: identifing a state of an automaticrepeat request control window for at feast one transceiver; estimating auser quality value for each available transmission mode based on thechannel parameter, the transmission capacity parameter, and the state ofthe automatic repeat request control window from at least onetransceiver; and selecting a transmission mode of the plurality ofavailable transmission modes, the selected transmission mode providingthe best user quality value; wherein the user quality value for eachavailable transmission mode is described by a corresponding user datathroughput (T_(i) ) for that transmission mode; and, wherein the step ofestimating the user data throughput (T_(i) ) is based on a protocol dataunit error rate (PER_(i)), a maximal data rate (R_(maxi)), atransmission time b and transmission frame length L, and the state ofthe automatic repeat request control window (R_(w) ) from at least onetransceiver.
 6. A transceiver having a controlling part, the controllingpart comprising: means for calculating a channel parameter based on alink quality; means for determining a transmission capacity parameterfor each of a plurality of available transmission modes; means foridentifying a state of an automatic repeat request control window in atleast one transceiver; means for estimating a user quality value foreach available transmission mode based on the channel parameter, thetransmission capacity parameter and the state of the automatic repeatrequest control window from at least one transceiver; and means forselecting a transmission mode of the plurality of available transmissionmodes, the selected transmission mode providing the best estimated userquality value; wherein the transmission capacity parameter is determinedby a maximal data rate (R_(maxi) ) provided in each availabletransmission mode, a transmission time b and transmission frame length Lprovided for transmitting data packets.
 7. The transceiver according toclaim 6, wherein the transceiver is part of a radio packet data system.8. The transceiver of claim 6, wherein the channel parameter for eachavailable transmission mode is a protocol data unit error rate(PER_(i)).
 9. The transceiver of claim 6, wherein, the state of theautomatic repeat request control window (R_(w) ) is determined by anoldest protocol data unit sequence number TxBoW, a newest protocol dataunit sequence number (TxToW), and a number of data packets (Nr) in awindow to be retransmitted.
 10. The transceiver of claim 6, wherein thestate of the automatic repeat request control window (R_(w) ) isdetermined by an oldest protocol data unit sequence number (RxBoW)expected by a receiver unit, a newest protocol data unit sequence number(RxToW) received by the receiver unit, and a number of data packets Nrin a window to be retransmitted.
 11. The transceiver according to claim6, wherein the user quality value for each available transmission modeis described by a corresponding user data throughput (T_(i) ) for thattransmission mode.